Table of Contents
5.1.5 Atmospheric Temperatures
Problems caused by low atmospheric temperatures:
Low atmospheric temperatures are usually ideal for engine operation. For example, the greater air density creates a greater mass flow, increasing performance without raising the gas temperatures, which would also raise the hot part temperatures.
However, low atmospheric temperatures can also have many undesirable effects on engine operation and even cause damage:
- Failure of auxiliary components such as pumps and regulators through heat strain problems between the housings and movable parts (pump rotors, valve gates, etc.) or through permanent structural changes (for example, changes in the shape of steels that have not been properly heat-treated-residual austenite conversion)
- Jamming of adjustment systems such as guide vane adjusters or adjustment cylinders.
- Rotors jamming due to clearance loss between the blade tips and housings (especially during start-up: affects the time interval until restart is possible).
- Increases the viscosity of fuel, oil, and hydraulic fluids, which increases the friction forces of cured movable components.
- Changes in material behavior through embrittlement; the notch impact strength of hardened and tempered and/or hardened steels (used in input and output shafts, etc.) may be greatly decreased, which can cause brittle fractures under special conditions such as a sudden increase in the power transfer or RPM.
- Failures of synthetic seals due to embrittlement and/or loss of elasticity (Fig. "Synthetic seals")
- Impairment of the ignition (Ref. 5.1.4-1) during engine start-up on the ground or (in small engines) restart at high altitude with less energy provided to the starter generator by cold batteries. This can decrease the start-up RPM and the pressure ratios in the combustion chamber, worsening the fuel atomization. In addition, the increased fuel viscosity results in larger droplets behind the fuel nozzle, making ignition of the fuel in the combustion chamber more difficult. The low evaporation rate and increased reaction inertia of the cold fuel make ignition more difficult and retard flame propagation (Fig. "Start-up behavior"). Even after the flame as stabilized, the poor combustion chamber efficiency combined with the higher power output demand (increased friction losses) can slow the acceleration of the engine considerably. Attempts to solve the problem by increasing the fuel flow can lead to overheating damage to the hot parts and/or compressor stalls (Ref. 5.1.5-2). With modern electronic control units it is possible to influence the starting process in such a way that the above problems are avoided without any special measures on the part of the operator.
Engines must start reliably under all conditions on the ground and at high altitude. This is true for long periods of low temperatures on the ground (Fig. "Takeoff") as well as for low temperatures and low air pressure at high altitude, and puts high demands on the combustion system. Typical requirements for start-up and operation are (Ref. 5.1.5-1):
- Temperatures below -50 °C accompanied by fuel viscosity of 12 centistokes
- Restarts at altitudes up to 11,000 meters for engines, 12,000 meters for APU'sInfluence on the life span of engine components (Ref. 5.1.5-3); important components of modern military engines usually have limited life spans. Factors affecting the life span are stationary and temporally changing temperature gradients - depending on the engine type. These are especially important for rotor components (thermal fatigue, Thermo Mechanical Fatigue =TMF), because direct the heat strain that overlays with the centrifugal force. With the calculated life spans, depending on the algorithms used, it was discovered that seasonal characteristics were of primary importance. If atmospheric temperatures are low, then the operating temperatures of the hot parts will also be lower, increasing the expected life span.
Icing in the fuel system can block valves, filters, and nozzles, possibly leading to fuel shortages in all engines. This dangerous situation can occur if an unallowably large amount of water is present in the fuel (Refs. 5.1.5-4 and 5.1.5-5). The presence of modern water separators in the fuel supply system on the ground usually prevents this type of dangerous situation. However, in 1998 a large cargo aircraft evidently crashed during takeoff due to fuel icing (Ref. 5.1.5-6).
Figure "Synthetic seals": Seals which contain synthetic parts (for example, O-ring seals 1 and 2, radial shaft seals 3, and flange seals 4) may lose the elasticity necessary for the sealing effect at low temperatures. This will not normally occur at the usual operating temperatures, and satisfactory operating behavior must be verified during cold runs as part of the engine certification process. However, Example "" shows that this does not make failures due to cold entirely impossible. Perhaps seals with unsuitable synthetic materials were mistakenly used in this situation.
Figure "Takeoff": This diagram shows a case of several almost simultaneous engine failures (in different aircraft of the same type). Atmospheric temperatures dropped to -15°C over night, cooling the aircraft accordingly. When an alarm start occurred the next morning, several aircraft could not takeoff because the thrust jet adjuster no longer functioned. An inspection revealed that in all the affected aircraft/engines, the hydraulic pumps of the thrust jet adjuster temporarily jammed. The bent fibers at the fractured area of the input shaft clearly indicated that the input shafts were violently sheared off at the predetermined breaking point through torsion overloads. The gear pumps had become unjammed by the time of the inspection. Signs of overheating (droplets of alloy components with a low melting point were found on the pressure disks, and the face surfaces of the pump gears opposite were tarnished) showed that a jam had actually occurred.
The jamming of the pumps was explained by the low pump temperature causing the various materials of the pump components spreading open the pump play. The cold hydraulic fluid (engine oil) further cooled the light metal housing (large heat strain) while the inner parts (pressure disks, gears, etc.) had high local temperatures during start-up, progressively increasing the jamming process. Evidently the certification process was not sufficiently realistic and did not simulate these conditions.
Example "Oil leakage at cold ambient temperature" (Ref. 5.1.5-7):
Excerpt: “Meanwhile, on 8 January the FAA issued an emergency airworthiness directive requiring pre-flight engine test runs and inspections of certain commercial airlines and business jets when the oil temperature is below freezing (32°F=0°C). The order affected 120 US aircraft…
During the cold snap, the FAA received three reports of inflight engine shutdowns affecting these aircraft. The FAA found that `starting these engines in very cold temperatures (below 32°F=0°C) could cause the starter shaft O-Ring seal to allow oil to leak from the engine's accessory gearbox.'
The FAA Airworthiness Directive required operators to perform engine check when the oil temperature has dropped below 32°F (special aircraft types)….operators must do a high-power leak check prior to flight on each of the two engines by running the engine for at least three minutes at takeoff and checking for oil leaks. Oil leaks cannot exceed two-10ths (ca. 0.23 l) of a quart per engine per hour or the aircraft is prohibited from further flight.”
Comments: It is interesting that the affected engine type (evidently a low-power engine) is already dangerously affected by oil losses of a fraction of a liter per hour. Because engines must verify their operability at temperatures far below freezing during the certification process, this is obviously not sufficient carried out.
Obviously the oil leak is the result of a temperature caused embrittlement of the seal elastomer (Fig. "Synthetic seals").
There have been problems even after 10 years, this time in connection with the introduction of a design improvement (Ref. 5.1.5-16)
Example "Jamming of variable stator" (Ref. 5.1.5-13):
Excerpt: “…(the OEM) has issued modified operational guidelines for some of its …series engines until repairs can be made to eliminate surges that have caused in-flight engine shutdowns on nine (two different aircraft types) engines…The OEM has determined that the surges are due to insufficient clearance of the variable stator synchronization system in the high-pressure compressor on cold days. All engine surges and subsequent shut downs occurred on the first flight of the day and during fall and winter months. The problem centers on engine stators, the compressor case flange and carbon fiber blocks called ring runners that are attached to the outside of the engine vane rings to keep the rings round. In cold engines, the stators to which the blocks are attached warm quickly because they are near the engine's hot gas flow. The compressor case flange warms comparatively slowly because it is nearer the cool, outside air. Due to the different rates of expansion of the stators and the compressor case flange, the carbon fiber blocks can be forced against the flange, causing binding. Once the binding takes place, the engine may surge, requiring shutdown…The corrective maintenance can be performed in the field, but the engine must be lowered from the aircraft wing to provide access to all of the block locations.”
Comments: This example (also see Example "Oil leakage at cold ambient temperature") shows that coordination of the heat strain-influenced tolerances is important for sufficiently safe operating behavior of the components. It is interesting that the time the damage occurred was the same: the first start-up after overnight standstill. The question is why these conditions were not sufficiently tested during development and certification of the engine.
Figure "Start-up behavior": The two top diagrams (Ref. 5.1.5-1) show the influence of the fuel temperature on the ignition of the fuel in the combustion chamber. Compared with highly aromatic fuels, the fuel JP4 has better start-up characteristics, as shown in the relatively short time it takes to ignite. This is also true for the time until the idling RPM is reached. This behavior is especially important for restarting engines or APUs at high altitudes. Delayed ignition loads ignition and starter higher and can increase the necessary maintenance effort (see Ill 5.1.5-3.2).
There are several effects that must be considered:
- The increased viscosity of the cold fuel worsens the spray pattern from the fuel nozzle; i.e. droplet size increases
- The evaporation of the cold fuel becomes worse
- Slower reaction rates during combustion
- Increased friction between bearings and glide surfaces
- Less power can be drawn from cold starter batteries
The bottom diagram (Ref. 5.1.5-2) shows the ignition range which must be maintained, dependent on the atmospheric temperature and fuel flow rate. If the amount of supplied fuel is too small, it will not ignite; if it is too large, then the pressure in the combustion chamber increases too rapidly, causing a compressor surge and possible consequential damages such as hot part overheating.
The climate influence affects also the LCF lifetime of the rotors. Ill 5.1.5-5 shows this exemplary for a modern fighter aeroengine (Ref. 5.1.5-14).
Figure "Ambient conditions" (Ref. 5.1.5-15): Starter and ignition device can be heavily loaded by low ambient temperatures. This can affect the necessary maintenance effort (see to this also Fig. "Start-up behavior").
Problems with high atmospheric temperatures
High atmospheric temperatures occur primarily during the takeoff phase, when high engine performance is required. They result in less dense air and a smaller mass flow, resulting in a corresponding drop in engine performance. In order to compensate for this power loss, the gas temperatures must be increased accordingly (exponential life span shortening of the hot parts), or the mass flow increased by injecting it with water or a water/methanol mixture. This causes condensation which reduces the temperature of the air flow and also increases mass.
Unusually high inlet temperatures can occur for various reasons:
- Weather-related atmospheric temperatures (hot day).
- Ingestion of recirculated hot gases in VTOLs or bad wind direction during standstill (for example, in helicopters, Fig. "Surge due to lack of oxygen").
- Ingestion of hot steam during catapult starts on aircraft carriers.
- Ingestion of weapon exhaust gases (rockets, machine guns).
As with low air pressure, increasing atmospheric pressure causes power losses (Fig. "Thrust" top). If both effects combine during aircraft takeoff (high power demand), then the engines are subject to high stress. If the prescribed limits (max. gas temperature, etc.) are maintained, then there is only a limited amount of power available (Ill. 5.1.5-3 bottom) above the attainable total pressure ratio. This situation is common at high-altitude airports in hot regions such as Quito, Columbia or Addis Abeba, Ethopia.
These problems may be compounded if the high temperatures recorded by the temperature sensors over the runway at the engine inlet do not correspond with the temperatures received from the tower (Ref. 5.1.5-8). This temperature can be several °C on hot days. There can also be misunderstandings if the atmospheric pressure around the standing aircraft is the “true” pressure, rather than the atmospheric pressure given by meteorological stations or the tower, which is atmospheric pressure corrected for sea level.
With military engines, it is possible that in serious cases extremely high engine output is required that can only be achieved by increasing the gas temperatures, even if the atmospheric temperature is high (Example "Problems caused by high ambient temperature"). Naturally, in this case a decrease in the life span of the hot parts is to be expected. In addition, unexpected phenomena such as high frequency vibrations in the afterburner system may occur and cause fatigue damage in a very short time.
High atmospheric temperatures pose an additional problem for electronic parts. Microprocessors and memory chips are especially prone to fail due to short- or long-term damage. For this reason, electronic engine control systems are often cooled (with cold fuel, etc., see Example "" ).
If the fuel temperatures become to high due to high atmospheric temperatures, it can result in internal oxidation and particle formation in the fuel. This can cause erosion of the injection nozzle with extensive consequential damages (Ref. 5.1.5-9).
Figure "Thrust": Increased inlet air temperatures lead to lower engine performance and less thrust (top diagram).
This can be explained by the decrease in the allowable total pressure ratio along with the increasing atmospheric temperature (Ref. 5.1.5-8). The reason for this is the decrease in the mass flow. Lower barometric pressure (due to a high-altitude runway, etc.) means that the air density will be lower, further limiting the allowable total pressure ratio (horizontal “PAM-Lines”). This also limits the amount of fuel that can be fed to the engine.
Example "Problems caused by high ambient temperature" (Ref. 5.1.5-11):
Excerpt: “The thrust loss experienced due to high ambient temperatures (15% at ISA+25°C conditions) was a primary problem facing …air-to-surface operations in the low level environment. The (aircraft type) is particularly performance critical at high all up weights and furthermore, high airspeeds were necessary for survivability and mission effectiveness.
….by allowing the engine to run up to 25°C hotter, this measure recovered approximately half of the thrust lost.
The up-rating of the engine increased the possibility of occurrences of a phenomenon known as reheat screech, which is a high frequency instability of the gas stream and can result in mechanical failure of the jet pipe liner in a matter of seconds.
…it was predicted that the useful lives of hot-end, non-group A parts, which includes blades could be reduced by as much as 50%. As most blades are lifed `on condition', a routine intra-scope inspection was introduced at 35 hour intervals.”
Comments: This concerns a light, twin-jet fighter plain in military missions in a desert environment with high atmospheric temperatures. The performance which was absolutely necessary for completing the flight missions was achieved by increasing the turbine inlet temperature without regard for the considerable life span reduction of the hot parts (Ill. 12.5-4).
Aside from this plausible effect, it is interesting that the high atmospheric temperatures indirectly affected the life span of the afterburner through unusual vibrations (Ill. 11.2.4-13).
Example "Active cooling of electronic control units" (Ref. 5.1.5-12):
The following excerpt is from an article regarding a certification program.
Excerpt: “Program officials have decided to add an active cooling system on the aircraft's engine control unit because of heating conditions that have been experienced on hot days.”
Comment: The cooling of the electronic aeroengine control was introduced during the development phase of the airplane. How this took place is not indicated in the available literature. However concerned may be a cooling with fuel.
Coolings of electronic/digital aeroengine control units with fuel are known at aeroengines in fighter aircrafts. This can determine the mounting location at the aeroengine, to minimize the risk of a damage in the case of containment (fracture of a fan blade).
Figure "Influence of climate" (Ref. 5.1.5-14): The climate in which an aeroengine is operated is of special significance for the lifetime of rotor components and hot parts. Naturally the climate plays also a role for other failure relevant influences like corrosion, the behaviour of oil and fuel and FOD risk (birds, ice). This is true as well for the failures to be expected as also for the repair effort respectively the logistics. This illustration shows exemplary the estimated effects for a fighter aeroengine. Concerned is an aeroengine type with low bypass ratio and two shafts. Exists an “engine monitoring system” , it must consider the climate influence.
The cyclic load of the aeroengine components is primarily determined by temperatures and pressures in the aeroengine, as well of the rotor speed. These depend again close from the ambient intake temperatures.
Thermal Fatigue (= TF, see chapter 12.6.2): In the displayed case, the material temperature of the trailing edge from the high pressure turbine rotorblades serves as aeroengine type specific lifetime indicator. It is primarily depending from the position of the powerlever and the intake temperature. With increasing intake temperature the material temperature rises in a first approximation linear in the temperature range of -50°C bis 50°C. Looking into the charts of the flight-envelope to be expected intake temperature at a related flight speed can be seen. With the Machnumber 1 in a similar flight height of about 4000 m at a polar day an ambient temperature of 20°C must be expected. In contrast at a tropics day there are about 60°C. This is a temperature difference of about 40°C (example 1). Example2 shows in an altitude of about 8000 m at the polar day - 5 °C, at the tropics day 25°C. This means a temperature difference of 30 °C. The TMF lifetime is reduced at a tropics day, correlating the used algorithms, about the factor 3 (diagram below).
The creep lifetime drops at a 30°C increase of material temperature to about 25 % (chapter 12.5). 60 °C mean usually more than the factor 10. In the present case a mean value of the factor 5 was evaluated.
The rotation speed depending LCF lifetime of the rotor due to the ambient temperature is difficult to evaluate. At cold days the aeroengine delivers, with the power lever demanded performance at lower rotation speeds. The rotation speed differences are in the range of max 10 %, rather lower 5 %. Because the rotation speed effects, the load of the rotor by the centrifugal force is quadratic, it must be estimated with an equal influence at the numbers of the lifetime influencing cycles. The design load in the case on hand is chosen, that only extreme cold conditions can be used for an increase of the lifetime.
5.1.5.1 Measures against damage due to high atmospheric temperatures
- Making seal parts from synthetic materials that do not undergo any unallowable changes due to temperature, especially no embrittlement in cold temperatures or aging at high temperatures; proper inspection and verification during certification and delivery; suitable behavior must be ensured even after long operating periods in contact with operating fluids such as oil or fuel.
- Metallic materials must be stable at operating temperatures, including those parts which are exposed to the atmospheric temperatures. For example, steels must not undergo any unallowable changes in volume due to belated conversions.
- Changes in properties important for operation, such as the notch impact strength of shafts, must be taken into account during the design phase.
- Auxiliary components and parts such as pumps and regulators for fuel and oil with tight tolerances and play must be tested to ensure safe operation at realistically large temperature differences.
- Sufficient start-up behavior of engines must be verified with different fuel types.
- The takeoff power of engines must be sufficient even at realistically high atmospheric temperatures and low pressures (takeoff from high-altitude airports on hot days) without unallowably affecting the designed life span of the hot parts.
References
5.1.5-1 I. Critchley, P. Sampath, F. Shum, “Cold Weather Ignition Characteristics of Advanced Small Gas Turbine Combustion Systems”, AGARD-CP-480 proceedings of the conference “Low Temperature Environment Operations of Turboengines (Design an User's Problems)”,
Chapter 9.
5.1.5-2 R.R. Pollack, “Control System Design Considerations für Starting Turbo-Engines During Cold Weather Operation”, AGARD-CP-480 proceedings of the conference “ Low Temperature Environment Operations of Turboengines (Design and User's Problems)”, Chapter 12.
5.1.5-3 R.W. Cue, D.E. Muir, “Climatic Considerations in the Life Cycle Management of the CF-18 Engine”, AGARD-CP-480 proceedings of the conference “ Low Temperature Environment Operations of Turboengines (Design and User's Problems)”, Chapter 15.
5.1.5-4 “Fuel System Icing Described As Major Hazard to Jet Planes”, periodical “Aviation Week”, December 8, 1958, page 27.
5.1.5-5 Aerospace Recommended Practice ARP 1401 “Aircraft Fuel System Component Icing Test”, Society of Automotive Engineers, Inc., 1979.
5.1.5-6 A.V. Velovich, “Multiple engine failure blamed for An-124 Irkutsk accident”, periodical “Flight International”, 17. Dec 97, page 435.
5.1.5-7 S. Shapiro, “Freeze and Flu”, periodical “Aerospace Risk” (ISSN 1465 5497), Issue 1, January 1999.
5.1.5-8 “The Aircraft Gas Turbine Engine and its Operation”, PWA Open Instr. 200, Pratt & Whitney Aircraft Group, page 160.
5.1.5-9 T. Edwards, “Prospects for JP-8+225, A Stepping Stone to JP-900”, AIAA 98-3532 paper from the “34 th AIAA/ASME/SAE/ASEE Joint Propulsion Conference& Exhibit”, July 13-15, 1998/Cleveland,OH.
5.1.5-10 W.T. Sawyer, “MFPG Technical Report No. 9”, Proceedings of the “Mechanical Failures Prevention Group”, 16th Meeting, Gaithersburg, Maryland November 2-4, 1971.
5.1.5-11 R.C. Sirs, “The Operation of Gas Turbine Engines in Hot And Sandy Conditions, Royal Air Force Experiences in the Gulf Conflict”, AGARD-CP-558 proceedings of the conference “Erosion, Corrosion and Foreign Object Damage Effects in Gas Turbines”, The Netherlands, 25-28 April 1004, Chapter 2.
5.1.5-12 B.A. Smith, “First 717 Delivery Shifts to September”, periodical “Aviation Week & Space Technology”, March 1, 1999, page 43.
5.1.5-13 “Pratt Issues JT9D Guidelines Pending Fix of surge Problem”, periodical “Aviation Week & Space Technology”, December 8, 1986, page 21.
5.1.5-14 R.W. Cue, D.E. Muir, „Climatic Considerations in the Cycle Management of the CF-18 Engine“, Conference Proceeding AGARD-CP-480, page15-1 up to15-14.
5.1.5-15 D.S. Breitman, F.K. Yeung, „Cold Start Development of Modern Small Gas Turbine Engines at Pratt & Whitney Canada”, AGARD-CP-480 Proceedings der Konferenz „Low Temperature Environment Operations of Turboengines (Design an User's Problems)“, chapter 9.
5.1.5-16 Federal Aviation Administration (FAA), „Aurworthiness Directives,; Rolls Royce Corporation AE 3007A and AE 3007C Series Turbofan Engines”, AD2007-24-05 vom 24.05.2005, page 1-12.